Extra heavy hydrocarbons, e.g. bitumen, represent a huge natural source of the world's total potential reserves of oil. Present estimates place the quantity of extra heavy hydrocarbon reserves at several trillion barrels, more than 5 times the known amount of the conventional, i.e. non-heavy, hydrocarbon reserves. This is partly because extra heavy hydrocarbons are generally difficult to recover by conventional recovery processes and thus have not been exploited to the same extent as non-heavy hydrocarbons. Extra heavy hydrocarbons possess very high viscosities and low API (American Petroleum Institute) gravities which makes them difficult, if not impossible, to pump in their native state.
A number of methods have been developed to extract and process heavy hydrocarbon mixtures. The recovery of extra heavy hydrocarbons from subterranean reservoirs is most commonly carried out by thermal enhanced recovery method such as steam assisted gravity drainage (SAGD), cyclic steam stimulation (CSS) or in situ combustion (ISC). In these methods the heavy hydrocarbon is heated and thereby mobilised, by steam in the case of SAGD and by a combustion front in the case of ISC, to flow to a production well from where it can be pumped to the surface facilities. The transportability of the viscous heavy hydrocarbon mixture recovered is conventionally improved by dilution with a lighter hydrocarbon.
The thermal recovery processes currently used suffer from inherent drawbacks. These include the consumption of vast amounts of energy, usually in the form of increasingly expensive natural gas, in the production of steam and the concomitant high carbon dioxide emissions which occur. Of course it has already been recognised in the energy industry that carbon dioxide emissions must be managed better.
One of the ways in which steam assisted gravity drainage methods have been improved as a means of recovering extra heavy hydrocarbons such as bitumen is through solvent facilitated start-up means or so-called ‘solvent soaks’. These methods accelerate start-up for steam assisted gravity assisted methods for the recovery of extra heavy hydrocarbons from a formation. Typically, such a method involves the following steps: forming a steam-assisted gravity drainage production well pair comprising an injection well and a production well within formation; soaking one or both of the wellbores of the well pair with a solvent (the pre-soaking stage); heating the wellbores of the well pair the pre-heating stage); injecting steam into the wellbores of the well pair (the squeezing stage); and then conducting steam-assisted gravity drainage production. Such methods are disclosed in WO-A-2012/121711, US 2013/0199779, WO-A-2015/000065 and U.S. Pat. No. 4,385,662.
There are, however, a number of problems that exist with these existing solvent soak SAGD production methods:
(a) They do not necessarily apply a pressure that is slightly greater than the mini-frac pressure (the fluid pressure required to create localized fractures in a formation when said fluid is injected into the formation) to the formation during the soaking period. As a consequence, mini-fractures/dilation in the near well region are unlikely to occur. Under such solvent soaking conditions, solvent penetration into the formation from a wellbore will only rely on the mobile water, which can be quite low in some reservoirs. With a low degree of mobile water in the reservoir, it significantly reduces the amount of solvent that can be pushed into the reservoir.(b) Without liquid solvent convective movement in the porous media, the existing technology utilises a diffusion process (which is a slow process) for solvent-bitumen mixing. This might result in a limited solvent penetration.(c) Squeezing solvent with steam during the solvent soaking period might preferentially push the solvent into an uncontrollable region, i.e., a region where the permeability is already high, thus making the solvent soaking less effective in the low permeability locations.
Various attempts have been made to address some of these problems, with varying degrees of success.
U.S. Pat. No. 6,769,486 is not a steam assisted gravity drainage production but it does disclose a solvent-soak process for recovery of heavy hydrocarbons. The process comprises injecting a viscosity reducing solvent of a fraction of said hydrocarbons into the reservoir at a pressure in the reservoir of above a liquid/vapour phase change pressure of a fraction of said solvent, the pressure in the reservoir also being sufficient to cause geomechanical formation dilation or pore fluid compression; allowing the solvent to mix with the hydrocarbons under pore dilation conditions; reducing the pressure in the reservoir to below the liquid/vapour phase change pressure of at least said fraction of said solvent thereby evincing solvent gas drive of said fraction of said hydrocarbons from said reservoir; and then, (d) repeating steps (a) to (c) as required. Unlike in SAGD, there is no injector/producer pair of wells.
CA 2778135 discloses an in situ bitumen recovery startup process that comprises injecting a solvent containing startup fluid into a first horizontal well, providing a pressure sink in a second horizontal well for pressure drive of the solvent toward the second well to mobilise the inter-well region, and establishing fluid communication between the first horizontal well and the second horizontal well.
Another example of a solvent soak system is disclosed in US 2011/0174488, in which solvent is injected into the horizontal injector well located in the heavy hydrocarbon reservoir to accelerate fluid communication between it and the horizontal production well with which it is in fluid communication. According to one embodiment, the solvent can be displaced into the reservoir using a displacing fluid such as a gas.
Each of these methods employs solvent soaking, heating and the application of pressure to assist the recovery of heavy hydrocarbons (although U.S. Pat. No. 6,769,486 is not a steam assisted gravity drainage method). These are stated to help to improve heavy hydrocarbon recovery from reservoirs. However, there is still a need for a more efficient solvent soak method for the recovery of heavy hydrocarbons such as bitumen from reservoirs.